Depending on your readiness (there are 3 different levels of readiness, I believe), different things happen (consequences in the ending for not being fully prepared! Who would've thought?!).
Shepard approaches the beam that leads to the Citadel.
Least prepared: Happens exactly the same as it is now, up until Shepard reaches the Illusive man. Since you weren't prepared, you see the entire fleet around you being decimated, and the Illusive man ends up controlling and indoctrinating Shepard before the Crucible can be activated. The Normandy is destroyed by Harbinger. Cutscenes are given, showing various locations over the next few hundred years, as the major races fall. The galaxy is defeated, and cycle continues.
Somewhat prepared: Happens exactly the same up until Shepard reaches the console. The fleet is taking heavy losses, and the Normandy and Harbinger both destroy each other. He activates the Crucible, killing all of the Reapers in the galaxy. He sits down next to Anderson, and dies next to him as he watches Reapers be destroyed. Cutscenes show what happened to the various races in the future, depending on if you helped them or not (i.e. not curing the genophage, but getting the krogans to help will result in them becoming extinct very quickly due to heavy losses and not being able to reproduce fast enough).
Fully prepared: Harbinger is busy fighting the fleet in orbit, so isn't there to stop Shepard from reaching the beam. Shepard fights through hordes of Reaper troops on his way to the control room. Shepard is able to fight the Illusive Man's ability to control him. Illusive Man dies, Anderson lives. The fleet is holding their own against the Reaper forces, and the Normandy destroys Harbinger with its thanix cannon. The crucible is activated (with this song playing in the background:
www.youtube.com/watch?v=OgBGWT8MiUM ), wiping out all of the Reapers in the galaxy. Cutscenes are shown on various worlds across the galaxy, as everyone cheers as they watch the Reapers drop dead. Shepard gives a heroic speech on Earth with all of his friends standing next to him. After that, it shows what happened to the various races in the future, depending on if you helped them or not.